This invention is directed generally to the monitoring of a flow of fluid, such as anhydrous ammonia (NH3), and more particularly to a novel apparatus for detecting a change, in the flowing fluid, from the liquid to the vapor phase.
Anhydrous ammonia (NH3) is extensively used as a fertilizer material in agriculture. Moreover, in relatively large-scale mechanized agriculture, relatively expensive and complex machinery is utilized to apply anhydrous ammonia to cultivated fields. This equipment is made somewhat more complex by the necessity of providing for proper and safe handling of anhydrous ammonia, including relatively large tanks for storage of a supply of the material, and well-sealed tubing, valves, metering devices and the like to carefully control the application thereof.
The anhydrous ammonia is most commonly applied by a so-called "knife", comprising a narrow blade-like implement which generally opens a narrow furrow or kerf and applies the anhydrous ammonia into the recently opened furrow through a narrow tube behind the blade. A plurality of such knives are generally provided in parallel, spaced sets on an elongate frame which is pulled by a suitable implement for applying material at the desired density.
It is important in such operations to keep track of the amount of fertilizer material applied, usually on a pounds per acre or some other weight or volume per area basis. In order to provide such metering of the flow of anhydrous ammonia, it is necessary to provide a flow metering device and a flow control device in line with the supply of anhydrous ammonia, and preferably prior to branching out to the one or more knives at which the material is applied. However, in order to accurately control and monitor the flow of anhydrous ammonia, the material must be in a liquid phase or state. That is, conventional flow meters and flow control devices are generally designed to operate with liquid materials rather than gaseous materials. Accordingly, all NH3 closed loop control systems employ some thermal exchange device in an effort to achieve a liquid phase at the metering point.
In general, systems for measuring the flow of this material assume that the metering or measurement area or volume is fixed and this volume is also assumed to be fully occupied by a flowing stream of liquid at all times, That is, a continuous or steady state situation is assumed. It is further usually assumed that the thermal exchange device has reliably provided substantially 100% liquid state material by converting any and all vapor into the liquid state just prior to the metering point.
However, it is apparent that any heat exchanger and/or other system of practical size and cost will only operate to convert a given proportion (i.e., less than 100%) of vapor to liquid at a given temperature. Beyond this practical limit, some vapor will pass through the flow meter resulting in some proportionate error in the measurement of flow.
While the power rating for a given heat or thermal exchanger is readily determinable, this information is not particularly useful in actual NH3 application operations. Rather, the primary matter of interest is maintaining the maximum vehicle speed over the ground consistent with maintaining the desired application density of the material. In large scale farming operations, it is important to optimize all operations, which in turn requires that a maximum speed of operation be attained in passing over the field for planting, fertilizing and cultivation procedures.
In theory, a maximum flow rate should be predictable, once one has determined a system's static and thermal losses. However, such losses depend upon the length, diameter, and condition of the piping and hosing on a given applicator, the existence and condition of couplers and valves, the condition of a supply tank and attendant plumbing and the nominal pressure and temperature of the supply tank, as well as knife injection pressure. These static and thermal system losses are therefore extremely difficult to predict and/or measure.
Moreover, it has been determined that even the thermal energy differences encountered from relatively bright sunlight as opposed to overcast days may be significant in effecting the thermal losses of a given thermal or heat exchanger. Accordingly, since energy losses per unit time are not readily predictable, the power rating of a given exchanger is not useful in determining the maximum flow capacity for a given anhydrous (NH3) application system. Thus, most operators must determine this from an essentially trial and error basis and by almost continuous observations to determine the actual maximum operating flow condition and hence optimum speed of operation with a given system. Needless to say, with relatively complex agricultural machinery, continuous observation by a single operator of not only the operating flow rates, but also the many other parts of the equipment which may require observation and checking, is a most difficult, if not an impossible proposition.
In order to remedy this situation, some systems have proposed various temperature differential measurements across the thermal exchanger and/or the monitoring of frequency output variations of a flow meter. While such methods are in theory workable, in practice the response times of such systems have proven much too long to provide any but a relatively coarse result, and greatly delayed corrections. That is, with these systems, a sufficient time
lag exists between the onset of the undesirable condition (i.e., excessive anhydrous ammonia in vapor phase) and a reliable indication of the condition, that the actual correction is only made after an improper rate of distribution has been in effect for some while.
We have found that surprisingly improved results may be obtained by visually observing the flow of NH3 through a section of transparent pipe in the vicinity of the flow meter. We have recognized that confined NH3 has an equilibrium temperature and pressure that must be physically satisfied at all times if the liquid phase is to be maintained. Moreover, we have found that the formation of vapor bubbles is readily observable for even minute deviations of temperature or pressure from the equilibrium point. Moreover, this bubble formation or "boiling" occurs almost instantaneously upon variation of the fluid temperature and pressure from this equilibrium point. Accordingly, we have discovered a very useful detection mechanism which gives a nearly zero time lag between onset of this undesirable condition and the onset of observable effects thereof.
We have further discovered that since the dielectric constant of liquid NH3 is generally 20 to 40 times greater than that of its vapor state, these bubbles are readily discernible by the use of electromagnetic waves in or near the visible spectrum. Accordingly, we have chosen to use readily available infrared radiation producing and detecting devices to monitor the flow of a stream of NH3 in a section of tubing or a fitting placed relatively near the flow metering point in the system. However other forms of radiation, e.g., ultrasonics, might also be used without departing from the invention in its broader aspects.